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Re: Is time an illusion?

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On 2017-09-26 20:18, Marcel Mueller wrote:
> On 25.09.17 22.30, Daniele Futtorovic wrote:
>> I'm trying to synchronise a process between two machines (actual, not
>> virtual). I require sub-millisecond precision.
> 
> What is the distance between these two machines?
> 
> If they are far away there is not even a physical definition of the term
> "the same time" within the required precision. The theory of relativity
> prohibits this. The maximum theoretical distance for let's say 0,1ms is
> 30km.
> However, you can still make more precise assumptions about time
> differences under certain conditions.

The problem space can be, at least as a working assumption, restricted
to a LAN.

I get your explanation; let me point out though that for my purposes, I
do not need the concept of "same time" -- only of same /interval/.
Which, from what has been said here and that I've read up on in the
meantime, indeed appears to be something I have to give up on, or at
least that I can't rely on perfectly (barring specialised equipment, as
you mention further on).

>> If so, seriously, what the heck? I'm aware I might have stumbled on a
>> very elementary truth here after so many years in the business, but I
>> must say, the idea itself feels deeply unsettling.
> 
> Welcome to real world. ;-)

... shattering the foundations of the virtual one. ;)

>> Existential musings aside, would any of you have suggestions as to how
>> tackle the issue?
> 
> Supply both computers with an atomic clock.
> Reasonable Rubidium clocks are not that expensive. The most challenging
> task is to synchronize this clocks and, of course, avoid power failures
> after that.
> 
> A GPS clock might also be an alternative. They have very good long term
> precision - basically limited by the Caesium clocks of the satelites -,
> but may suffer short term stability because of atmospheric effects or
> interference.
> 
> ntp, as recommended, is /not/ sufficient to synchronize within
> microsecond precision.

Yeah... no. The need for specialised equipment would be prohibitive for
the intended uses. It's good to know it would be an option,
theoretically speaking, but in this case, that's definitely out of the
question.

As mentioned elsethreads, I have for now fixed my problem by
synchronising my clocks periodically (every 30s for now). It seems to
work well enough for the usage, at least at low latencies. Like any
network-based synchronisation protocol, its accuracy will depend on the
symmetry between the upward and downward leg of the round-trip, but I'll
have to see how bad it can get.

-- 
DF.

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Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-25 22:30 +0200
  Re: Is time an illusion? Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-09-25 18:02 -0400
    Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-26 00:16 +0200
      Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-25 19:18 -0400
      Re: Is time an illusion? Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-09-26 15:37 -0400
        Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 01:13 +0200
          Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-26 21:00 -0400
            Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 04:26 +0200
              Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-26 23:32 -0400
                Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-28 10:58 +0200
                Re: Is time an illusion? Jerry Stuckle <jstucklex@attglobal.net> - 2017-09-28 12:50 -0400
            Re: Is time an illusion? Eric Douglas <e.d.programmer@gmail.com> - 2017-09-27 05:25 -0700
            Re: Is time an illusion? Arne Vajhøj <arne@vajhoej.dk> - 2017-09-30 22:57 -0400
  Re: Is time an illusion? Arne Vajhøj <arne@vajhoej.dk> - 2017-09-25 20:22 -0400
  Re: Is time an illusion? Marcel Mueller <news.5.maazl@spamgourmet.org> - 2017-09-26 20:18 +0200
    Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 01:30 +0200
      Re: Is time an illusion? lewbloch@gmail.com - 2017-09-28 10:52 -0700
  Re: Is time an illusion? Eric Douglas <e.d.programmer@gmail.com> - 2017-09-26 12:59 -0700
    Re: Is time an illusion? Daniele Futtorovic <da.futt.news@laposte.net> - 2017-09-27 01:42 +0200
  Re: Is time an illusion? lewbloch@gmail.com - 2017-09-28 10:49 -0700
    Re: Is time an illusion? Robert Klemme <shortcutter@googlemail.com> - 2017-09-28 22:33 +0200

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